1. Field of the Invention
The present invention relates to a broadband wireless access system, and more particularly, to a method of mapping ranging channels corresponding to uplink zones where a mobile station performs ranging and ranging opportunities, and a method for performing ranging in a mobile station and an apparatus for performing the method.
2. Discussion of the Related Art
The main standard defined by the IEEE 802.16 task group includes IEEE 802.16-2004 called fixed WiMAX and IEEE 802.16e-2005 (16e) called mobile WiMAX. The IEEE 802.16e-2005 has been finally approved from the IEEE on December of 2005. The standard based on mobile WiMAX of a current version includes IEEE 802.16-2004, IEEE 802.16e-2005 (this document includes Corrigenda of IEEE 802.16-2004), and IEEE 802.16-2004/Corrigenda2/D8. Currently, standardization of IEEE 802.16m (16m) for mobile WiMAX of next version is in progress by TGm within the IEEE 802.16 task group.
Initial ranging means a procedure of controlling transmission parameters (frequency offset, time offset, transmission power) for uplink communication between a mobile station and a base station during an initial network registration. After performing the network registration procedure, the mobile station performs periodic ranging to continue to maintain uplink communication with the base station. In addition, examples of ranging include handover ranging for simplifying a procedure during a handover operation of a mobile station and bandwidth request ranging performed during an uplink bandwidth request procedure when a mobile station desires to transmit data.
In a broadband wireless access system, a CDMA code (or ranging preamble) set that can be used for ranging depending on ranging types and a region for transmitting CDMA codes are allocated through a channel (for example, UL-MAP), which broadcasts system information, by a network. Accordingly, to perform handover ranging, a specific mobile station selects a specific code from CDMA codes for handover ranging and transmits the selected code to the network through initial ranging and handover ranging regions to request ranging. In this case, the network can identity a type of ranging through the received CDMA code and an interval where the CDMA code is transmitted.
In the IEEE802.16m system, examples of a ranging channel include a synchronized ranging channel (S-RCH) for ranging performed by a synchronized mobile station and a non-synchronized ranging channel (NS-RCH) for ranging performed by a non-synchronized mobile station. Also, a bandwidth request channel exists in the IEEE802.16m system to request an uplink band when a mobile station desires to transmit data. The ranging channels (S-RCH and NS-RCH) and the bandwidth request channel (BRCH) are used to mean ranging opportunity and BR opportunity, respectively.
The ranging opportunity is used for a ranging acknowledgement response (AAI_RNG-ACK) message for notifying a mobile station whether a base station has received a ranging code received during a ranging procedure, and a CDMA allocation map information element (CDMA Allocation A-MAP IE) for transferring resource allocation information for the successfully received ranging code to a mobile station that has transmitted the corresponding code.
At this time, different types of base stations transmit ranging channels and code allocation information in accordance with their respective manners. For example, a base station that supports WirelessMAN-OFDMA with FDM-based UL PUSC zone and a base station having narrow coverage, such as a femto cell, transfer ranging related information to the mobile station through a super frame header (SFH) which is an independent channel type. The other base stations (e.g., macro base station, relay station, and macro hot-zone) transfers different kinds of ranging related information to the mobile station through a super frame header (SFH) and a system configuration descriptor (AAI_SCD) message, wherein the SFH is an independent channel type and the system configuration descriptor message is a media access control (MAC) message type. The ranging channels and the code allocation information are transferred to the mobile stations at different times when the SFH and the system configuration descriptor (AAI_SCD) message are used at the same time.
In this case, the mobile station may perform initial ranging or handover ranging after receiving the SFH only even though it does not receive the AAI_SCD message. However, if the ranging channel allocated through the AAI_SCD message is first allocated on time axis and frequency axis, the mobile station that has not received the AAI_SCD message does not know the presence of the ranging channel.
For example, if the physical location of the ranging channel transferred through the AAI_SCD message is prior to that of the ranging channel transferred through the SFH on the time axis/frequency axis in one frame, the ranging channel of the AAI_SCD message is mapped into ranging opportunity index 0, and the ranging channel of the SFH is mapped into ranging opportunity index 1. At this time, although the ranging channel selected by the mobile station for initial ranging or handover ranging has actually ranging opportunity index of 1, since the mobile station has not received the AAI_SCD message, a problem may occur in that the mobile station recognizes the corresponding ranging channel as ranging opportunity index of 0.
Accordingly, a mapping rule of opportunity index of the ranging channel on the time axis/frequency axis as described above is not desirably required. A method of mapping physically allocated ranging channels into ranging opportunity should be defined newly.